1. Field of the Invention
The present invention relates to a collector mirror exchanging apparatus and a collector mirror exchanging method to be used to exchange a collector mirror for collecting extreme ultra violet (EUV) light emitted from plasma in an extreme ultra violet light source apparatus.
2. Description of a Related Art
In recent years, as semiconductor processes become finer, photolithography has been making rapid progress to finer fabrication. In the next generation, microfabrication of 100 nm to 70 nm, or even 50 nm or less will be required. Accordingly, in order to fulfill the requirement for microfabrication of 50 nm or less, for example, exposure equipment is expected to be developed by combining an EUV light source generating EUV light with a wavelength of about 13 nm and reduced projection reflective optics.
As the EUV light source, there are three kinds of light sources, which include an LPP (laser produced plasma) light source using plasma generated by applying a laser beam to a target (hereinafter, also referred to as “LPP type EUV light source apparatus”), a DPP (discharge produced plasma) light source using plasma generated by discharge, and an SR (synchrotron radiation) light source using orbital radiation. Among them, the LPP light source has the advantages that extremely high intensity close to black body radiation can be obtained because plasma density can be made considerably larger, that light emission of only the necessary waveband can be performed by selecting the target material, and that an extremely large collection solid angle of 2π steradian can be ensured because it is a point light source having substantially isotropic angle distribution and there is no structure surrounding the light source such as electrodes. Therefore, the LPP light source is thought to be predominant as a light source for EUV lithography requiring power of several tens of watts.
For example, EUV light is generated in the LPP type EUV light source apparatus under a principle as follows. That is, a target material is supplied into a vacuum chamber by use of a nozzle, and this target material is irradiated with a laser beam to be excited and turned into plasma. Light with various wavelengths including extreme ultra violet (EUV) light is emitted from plasma thus generated. Then, the EUV light is reflected and collected to be emitted to an exposure unit by use of a collector mirror (light collecting mirror) which selectively reflects light with a desired wavelength (13.5 nm, for example) therein.
As a collector mirror for collecting EUV light with a wave length near 13.5 nm, for example, a mirror is used having a reflecting surface on which thin films of molybdenum (Mo) and silicon (Si) are alternately deposited. Typically, the number of the deposited layers of the Mo/Si thin film ranges up to several hundred. Also, smoothness and a shape of the reflecting surface of the collector mirror should be controlled with an extreme preciseness, in order to increase a collecting efficiency of the EUV light. Therefore, the collector mirror is extremely expensive.
Further, the collector mirror is easily damaged by flying particles such as high speed ions and neutrons emitted from a target material turned into plasma. Therefore, the collector mirror is disposed as far as possible from a plasma generating point (light emitting point). Also, a collecting solid angle of the collecting mirror should be made larger for increasing output power of the EUV light, and therefore, the collector mirror becomes inevitably larger in radius and accordingly becomes heavy.
Exchanging of such a collector mirror is performed manually. However, it is considerably troublesome to exchange a heavy collector mirror and, if the operation failed, the failure probably would lead to a breakage of the collector mirror and an injury to an operator. Since the collector mirror is extremely expensive, resulting damage thereof is considerable. Therefore, a method of exchanging collector mirrors safely and easily is desired.
As a relating technology, Japanese Patent Application Publication JP-P2004-103961A discloses a mirror holding unit including a mirror and an elastic member for elastically coupling the mirror to a hatch which is openably provided on a wall of a chamber capable of generating reduced-pressure environment, in order to make it easy to exchange an illuminating optical system and keep an initial alignment accuracy, and further make short a time required for the exchange to improve exposure throughput.
In JP-P2004-103961A, however, weights of the collector mirror, the hatch of the chamber to which the mirror is attached, and the supporting member (elastic member) for coupling the mirror to the hatch are not taken into consideration. That is, although a heavy weight should be supported when fixing these parts to the wall of the chamber, no means for supporting the heavy weight is disclosed.